<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	 <script>
        function a () {
            this.num = [1,2,3];
        }
        a.prototype.say = function(){
            alert(this.num)
        };
        function b () {
            a.call(this);
        }
        b.prototype = new a();
        var c = new a;
        var d = new b;

        d.num.push(4);
        console.log(c.num) // 123
        console.log(d.num) // 1234
    </script>
</body>
</html>